This invention relates generally to the field of generating activated gas containing ions, free radicals, atoms and molecules and to apparatus for and methods of processing materials with activated gas.
Plasma discharges can be used to excite gases to produce activated gases containing ions, free radicals, atoms and molecules. Activated gases are used for numerous industrial and scientific applications including processing solid materials such as semiconductor wafers, powders, and other gases. The parameters of the plasma and the conditions of the exposure of the plasma to the material being processed vary widely depending on the application.
For example, some applications require the use of ions with low kinetic energy (i.e. a few electron volts) because the material being processed is sensitive to damage. Other applications, such as anisotropic etching or planarized dielectric deposition, require the use of ions with high kinetic energy. Still other applications, such as reactive ion beam etching, require precise control of the ion energy.
Some applications require shielding the material being processed from the plasma because the material is sensitive to damage caused by ions or because the process has high selectivity requirements. Other applications require direct exposure of the material being processed to a high density plasma. One such application is generating ion-activated chemical reactions. Other such applications include etching of and depositing material into high aspect ratio structures.
For some applications, activated gases need to be generated at or near the surfaces to be treated, because of the high reactivity and short lifetime of the gases. One example is atomic fluorine, which can be used to clean chemical vapor deposition (CVD) chambers for deposition of thin films onto substrate surfaces. CVD chambers need to be routinely cleaned, in order to remove the deposits that build up on the surfaces of chamber parts other than the substrate surfaces. Whereas wet cleaning of the chambers is labor intensive and hazardous to the workers, cleaning the chamber with atomic fluorine generated by a plasma source allows the deposits to be removed without opening the chamber to atmosphere, improving tool productivity and working conditions. Typical source gases for atomic fluorine include perfluoro compounds (PFCs) such as NF3, CF4, CHF3, C2F6, and C4F8. A high dissociation rate of the PFCs is important to reduce their emission to the environment and to improve process throughput. Another example is photoresist removal in microelectronics fabrication. After pattern generation, photoresist is removed by exposing the wafer surface to atomic oxygen generated by a plasma source. Atomic oxygen reacts rapidly and selectively with photoresist, allowing the process to be conducted in a vacuum and at relatively low temperature.
Plasmas can be generated in various ways including DC discharge, radio frequency (RF) discharge, and microwave discharge. DC discharges are achieved by applying a potential between two electrodes in a gas. RF discharges are achieved either by electrostatically or inductively coupling energy from a power supply into a plasma. Parallel plates are typically used for electrostatically coupling energy into a plasma. Induction coils are typically used for inducing current into the plasma. Microwave discharges are achieved by directly coupling microwave energy through a microwave-passing window into a discharge chamber containing a gas. Microwave discharges are advantageous because they can be used to support a wide range of discharge conditions, including highly ionized electron cyclotron resonant (ECR) plasmas.
RF discharges and DC discharges inherently produce high energy ions and, therefore, are often used to generate plasmas for applications where the material being processed is in direct contact with the plasma. Microwave discharges produce dense, low ion energy plasmas and, therefore, are often used to produce streams of activated gas for xe2x80x9cdownstreamxe2x80x9d processing. Microwave discharges are also useful for applications where it is desirable to generate ions at low energy and then accelerate the ions to the process surface with an applied potential.
However, microwave and inductively coupled plasma sources require expensive and complex power delivery systems. These plasma sources require precision RF or microwave power generators and complex matching networks to match the impedance of the generator to the plasma source. In addition, precision instrumentation is usually required to ascertain and control the actual power reaching the plasma.
RF inductively coupled plasmas are particularly useful for generating large area plasmas for such applications as semiconductor wafer processing. However, prior art RF inductively coupled plasmas are not purely inductive because the drive currents are only weakly coupled to the plasma. Consequently, RF inductively coupled plasmas are inefficient and require the use of high voltages on the drive coils. The high voltages produce high electrostatic fields that cause high energy ion bombardment of reactor surfaces. The ion bombardment deteriorates the reactor and can contaminate the process chamber and the material being processed. The ion bombardment can also cause damage to the material being processed.
Faraday shields have been used in inductively coupled plasma sources to contain the high electrostatic fields. However, because of the relatively weak coupling of the drive coil currents to the plasma, large eddy currents form in the shields resulting in substantial power dissipation. The cost, complexity, and reduced power efficiency make the use of Faraday shields unattractive.
Plasma sources can also be used to remove harmful gases from gas streams. Concern over global warming has driven semiconductor and other manufacturing industries to reduce their emission of PFCs. Conventional methods of abating environmental pollutants typically utilize thermal techniques which break down pollutant molecules by burning the gases at a high temperature. Due to the thermal stability of many of the PFCs, however, the thermal techniques are not very effective, and can be very expensive.
The invention relates to an apparatus for generating excited gases containing ions, free radicals, atoms and molecules, and for abating hazardous compounds.
In one embodiment, the invention provides an improved toroidal low-field plasma source with an ignition control circuit that allows plasma ignition within a wider range of gas conditions than are permitted generally by prior art plasma sources. In another embodiment, the invention improves the power efficiency of a toroidal low-field plasma source by automatically adjusting the power delivered to the plasma based on the load to the power supply. In another embodiment, the invention improves the dissociation and abatement efficiencies of a toroidal low-field plasma source by providing a gas mixing device.
In one embodiment, the invention provides a method for operating a plasma source over a wider pressure range than generally allowed by prior art plasma sources. In another embodiment, the invention provides a method for increasing the output of atomic species from a plasma source. In another embodiment, the invention provides a method for increasing the plasma etch rate of organic materials.
In another embodiment, the invention provides an apparatus and method for efficiently removing PFCs and other hazardous gaseous compounds from effluent gas streams, by converting the hazardous compounds into scrubbable products.
Accordingly, the present invention features an apparatus for dissociating gases into a plasma that includes a plasma chamber. In one embodiment, the apparatus includes a process chamber that is coupled to the plasma chamber and positioned to receive reactive gas generated by a plasma in the plasma chamber.
The apparatus also includes a transformer having a primary winding and having a magnetic core surrounding a portion of the plasma chamber. The apparatus also includes a switching power supply. In one embodiment, the switching power supply includes a switching semiconductor device that is directly coupled to a voltage supply. The output of the switching semiconductor device can be directly coupled to the primary winding of the transformer. The switching semiconductor device can be a switching transistor.
In one embodiment, the apparatus includes a free charge generator which assists the ignition of a plasma in the chamber. In an embodiment, an electrode is positioned in the chamber to generate the free charges. In another embodiment, an electrode is capacitively coupled to the chamber to generate the free charges. In another embodiment, an ultraviolet light source is optically coupled to the chamber to generate the free charges.
In one embodiment, the apparatus includes a circuit for controlling the ignition of the plasma in the chamber. In one embodiment, the ignition control circuit comprises a resonance capacitor which forms a resonant circuit with the switching power supply at the switching frequency of the switching power supply. The resonant circuit outputs a resonance voltage on the primary winding. The ignition control circuit also comprises an electronic control circuit which controls ignition of the plasma by regulating the current in the primary winding.
In another embodiment, the ignition control circuit comprises a resonance capacitor connected to the primary winding so as to form a resonant circuit at a particular frequency. The ignition control circuit also comprises an electronic control circuit which controls ignition of the plasma by tuning the cycle rate of the switching power supply to the resonant frequency prior to, during, and after ignition.
In one embodiment, the apparatus includes a power control circuit for adjusting the power delivered to the plasma based on the size of the plasma load. In one embodiment, the power control circuit comprises an electronic control circuit that includes a current comparator and a microprocessor. The control circuit adjusts the output power of the switching power supply based on the size of the load, by varying the duty cycle of the switching power supply.
In another embodiment, the power control circuit comprises a constant-current RF switching power supply that includes an inductive element connected in series with the plasma chamber and the transformer. The inductive element maintains current through the load at about the value of the initial inductor current, thereby adjusting power in the switching power supply based on the size of the load.
In one embodiment, the apparatus includes a gas mixing device that improves the dissociation and abatement efficiencies of the plasma source.
The present invention also features a method for dissociating gases into a plasma and controlling plasma ignition. The method includes providing a plasma chamber, and a transformer having a primary winding and having a magnetic core surrounding a portion of the chamber. The method further includes coupling a resonance capacitor to the primary winding and to a switching power supply so that the resonance capacitor and the switching power supply form a resonant circuit at the switching frequency of the switching power supply. The method includes placing a resonance voltage on the primary winding, and switching off the resonant circuit from the primary winding subsequent to ignition of the plasma. The method also includes the step of regulating current in the primary winding during and after the resonance voltage phase, and after plasma ignition.
Alternatively, the method for ignition control according to the present invention comprises the step of coupling a resonance capacitor to the primary winding and a switching power supply so that the resonance capacitor and the switching power supply form a resonant circuit at a predetermined frequency. The method also includes the step of tuning the cycle rate of the switching power supply to control plasma ignition.
The present invention also features a method for controlling power of the toroidal low-field plasma source of the present invention. In one embodiment, the method includes varying the duty cycle of the switching power supply, thereby adjusting the average output power of the switching power supply based on the size of the plasma load.
In another embodiment, the method for controlling power includes providing a constant-current RF switching power supply that contains an inductive element connected in series with the plasma load. The method includes maintaining current though the load at about the value of the initial inductor current so that power in the switching power supply is adjusted based on the size of the load.
The present invention also features a method for operating a toroidal low-field plasma source over a wider pressure range than generally allowed by prior art plasma sources. The present invention also features a method for increasing the output of atomic species from a toroidal low-field plasma source. The method includes adding a noble gas to a feed gas, before the gases are fed to the plasma source.
The present invention also features a method for increasing the plasma etch rate of organic materials such as photoresists. The method includes feeding a mixture of a noble gas with oxygen and nitrogen into a plasma source, and raising the pressure in the process chamber to above about 50 torr.
The present invention also features a method for abating toxic gases with a toroidal low-field plasma source. The method includes activating a PFC in the plasma chamber. The method further includes converting the PFC into a scrubbable product and removing the scrubbable product from the effluent gas stream in a scrubber.
The present invention further features a method for cooling the plasma chamber. The plasma chamber can be cooled by one or more cooling plates which are mechanically attached and thermally bonded to the plasma chamber. A cooling fluid flows inside the cooling plates, and the fluid is not in contact with the plasma chamber itself. Separating the plasma chamber from the cooling fluids allows the plasma chamber to be made of materials that are compatible with the reactive plasma regardless of their compatibility with the cooling fluids.